Hook to /panel/login "template" to customize or add HTML, from plugin

The CMS Questions
1

I’m trying to customize the /panel login.
I know I can do that from config.php with a tiny line of config code and a panel.css file, to do things like change colors or fonts, etc.

// config.php
return[
    'panel' => [
        'css' => 'assets/css/panel.css'
    ]
]

But now I would want to add custom template code to the panel (specifically the /panel/login) page.
And I’d like to do this in a dynamic way, not add a CSS file, but add a template code, for example some HTML to include a custom div or something.

Let’s say the content of a field in the site panel. For the sake of an example if I want to add a customizable button color:

<style>

k-login-button.k-button {
–-theme-color-back: <?= $site->loginColorBack() ?>;
}

</style>

How would I include/inject/hook that into the panel login page, and do it from a plugin?
Any hook I tried did not do anything.

Any tips or direction welcome. Thanks!